ELI5: Why can we feel a fan blowing air from the front, but barely feel it sucking at the back?

Read the Story

Show Top Comments

Like everyone is saying, it’s a matter of different cross-sectional areas and equal volumes causes a velocity differential. But I feel like that’s not really answering ops question. Given a perfectly symmetric fan blade (front to back) why would it be different? Essentially, it’s because while the blades are actively pushing air in a forward direction through direct application of force, there’s no way to do the same to “pull” air. The only way to pull air is to create a low pressure zone which by it’s very nature has omni-directional influence. Edit: I just realized what sub I’m in. Think about it like pushing your hand through sand or snow. You can actively push the sand forward and it will clump up and move a bit ahead of your hand mostly in one direction. But you can’t pull the material along with you. Instead you create a gap (low pressure zone) that fills in from sand/snow slumping I’m from the sides. Fans do something similar with air. They can push air forward but can’t pull air from the back. Instead they create a gap which air “slumps” into. Or picture sand falling down an hour glass. You can see and you could feel the sand quickly falling in a narrow stream, but you can barely notice or feel movement in the top section. Edit2: Because this has gained some traction people have pointed out errors in this explanation. I appreciate that and I want to be very clear that every analogy in physics is usually wrong as the actual mathematics is far more complex. Any thought experiment is only the first stepping off point into understanding the world around us. As you gain a deeper understanding you can leave behind every earlier approximation. Please read through the disagreements and oppositional arguments below to gain further insight into this phenomenon.


Outflow is getting pushed in one direction while inflow is grabbing from everywhere. That’s why a vacuum can go from doing nothing to being stuck on your hand, drapes, carpet, etc. Goes from a small pulling force all over to all the pulling force being just at the opening.


Imagine the airflow looking kind of like a mushroom, with a very wide top (suction) and a very narrow foot, the push.


The air pushed out the front maintains a tighter stream than the air being pulled in the back, which means it has a greater velocity. The total volume of air moving is the same, but in the back it’s being pulled from a much greater area. Basically you can think of it pushing a stream forward that tends to stick together for a distance like water from a hose. However, at the back you just have a low pressure zone where air is being pulled from the top, bottom, sides, and rear to fill in space where that air was removed.


Say theoretically you have a fan that sends 1 cubic meter of air per second, let us say it does this by sending it at 1 meter per second through a 1 square meter of area. The behind of the fan will have to bring the same amount of air per second than the output you have, so it has to send 1 cubic meter of air per second. But the behind of the fan is not limited by the area the blade covers, it can come from pretty much anywhere so it has a much bigger area. So if the area air can get into the fan is 4 square meters and only 1 cubic meter goes in per second then the speed of the air behind it has to be 0.25 meters per second,so the air doesn’t move fast behind the fan. Think of the fan sort of like a wind funnel, the area to get into a funnel is way bigger than the area to get out, you are slowly shoving the air into the funnel and it comes out the other end going way faster because all the air that got in needs to get out at the same rate. Just like when you press your finger on the water hose, the water needs to speed up because it needs to go at the same rate it was going before but through a smaller area, the fan concentrates the air that should have otherwise gone through a bigger area and pushes it through a smaller one at higher speeds.